1. Field of the Invention
This invention relates to a method for epitaxial growth and an apparatus for the epitaxial growth. More particularly, it relates to a method for epitaxial growth by means of a so-called heterogeneous reaction and an apparatus therefor.
2. Description of the Prior Art
Heretofore, the method for forming, on a semiconducting substrate of a single crystal, e.g. silicon, a thin film of a semiconductor single crystal, a species the same as or different from the substrate, has extensively utilized epitaxial growth for the production of a semiconductor device.
Control of the film-forming conditions is an extremely important factor from the viewpoint of forming a single crystal. The method has been known to be one of the most difficult of all the techniques that are available for the production of such semiconductor devices such as the LSI.
JP-B-43-21,367 discloses a primitive method for epitaxial growth by a heterogeneous reaction. This method of growth requires the temperature of a source substrate to be higher than that of the growth substrate for ensuring effective mass transfer. As shown in FIG. 1, therefore, a silicon substrate as a source substrate 3 is placed on a source substrate holder 2 which is provided with a heater and a silicon substrate as a substrate 5 for growth is opposed to the source substrate 3 across a prescribed gap.
Then, by causing a gas composed of bromine and a hydrogen compound thereof to flow through the gap between the silicon substrates 3 and 5, the source silicon substrate 3 is etched and, at the same time, the reaction product resulting from the etching is deposited on the silicon substrate 5 for growth.
At this time, the temperature of the substrate 5 for growth is maintained within a prescribed range lest the rate of growth should decline. For this reason, the two substrates are brought as close to each other as possible. If too close to each other, the result will be precipitation thereon of the reaction product from the etching. Therefore, they must be kept a proper distance from each other. A spacer 4 is interposed between the two substrates 3 and 5 for the purpose of keeping them at the proper distance.
After the growth in a given cycle is completed and before the growth in the subsequent cycle is started, the distance between the two substrates may require readjustment. It is necessary in this case that the spacer which has shortened be replaced or that the source substrate which has worn thin be replaced.
Though the method of growth mentioned above has been utilized for elucidating the fundamental mechanism of epitaxial growth, it has never been reduced to commercial operation because it cannot be adapted for mass production. This circumstance has led to the development of the CVD method in which a reaction product originating solely in a reaction gas is caused to be deposited on a substrate for growth. This CVD method has been the leader of all the existent methods for epitaxial growth. The plasma-enhanced CVD method and the light-assisted CVD method have been proposed as specific versions of the CVD method and the use of a compound such as the disilane (Si.sub.2 H.sub.6) for the CVD method has been also proposed. None of these methods has yet reached the level of practicability. At present, the pyrolytic CVD method is used as the most practical version of the CVD method.
The pyrolytic CVD method basically comprises heating a silicon substrate, for example, in an atmosphere of hydrogen at an elevated temperature of not less than 1000.degree. C., supplying a reaction gas such as SiH.sub.4, SiH.sub.2 Cl.sub.2, or SiHCl.sub.3 in combination with hydrogen onto a substrate for growth thereby causing such reactions as are shown below to occur on the surface of the substrate for growth and inducing the formation of a silicon film. This formation of the film must satisfy the following film-forming conditions:
(1) The reaction gas must be heated to a temperature higher than the reaction temperature. PA1 (2) Neither a natural oxide film nor any defiling substance can be present on the surface of the substrate. PA1 (3) The Si atoms formed by the decomposition of the reaction gas and deposited on the substrate should possess thorough mobility to permit formation of a single crystal seed. PA1 (1) Consequent increase in size of the apparatus, PA1 (2) Extreme difficulty encountered in consequence of the increase in size of the apparatus in attaining control of the pattern of gas flow and adjustment of the geometric shape of reactor and consequent possibility of degrading the uniformity of film thickness and film quality within and between the individual wafers, PA1 (3) Huge consumption of hydrogen gas for improved uniformity of film thickness and film quality and rise of energy cost of heating and ready dispersion of temperature distribution particularly in the case of a batch apparatus, PA1 (4) Degradation of throughput in the case of an apparatus of the sheet feed type, and PA1 (5) Rise of production cost per wafer.
For the purpose of removing the natural oxide film from the surface of the substrate, the practice of subjecting the substrate to a high-temperature treatment in an atmosphere of hydrogen is now resorted to.
The apparatuses for epitaxial growth which are applicable to the method for growth mentioned above are classified by the shape of reaction chamber as the vertical type, the horizontal type, the barrel type, the cluster type, or other. They are also classified by the method of heating as the resistance heating type, the high-frequency heating type, the lamp heating type, or other. By the method of wafer treatment, they are classified as the sheet feed type, the batch type, or other. In conformity with the trend of semiconductor devices toward increasingly high densification (number of components per chip) and integration, the wafers of recent manufacture predominantly have a diameter of 200 mm. It is expected that the age of wafers with a diameter of 300 mm will arrive in the near future. In this connection, the problems which confront the feasibilization of an apparatus for epitaxial growth or a method for epitaxial growth which can be adapted for the expected increase of diameter of wafers are now under study in search of a solution.
Adaption for the prospective increase in the diameter of the wafers, poses the following problems for the CVD apparatus described above:
Since the apparatus under discussion uses a gas such as SiH.sub.4, SiH.sub.2 Cl.sub.2, SiHCl.sub.3, or SiCl.sub.4 as the reaction gas, it requires full attention from the viewpoint of ensuring safety and preventing corrosion.
With a view to coping with the various problems confronted by the CVD method mentioned above, the primitive method for epitaxial growth which resorts to the heterogeneous reaction has come to attract renewed attention. To adapt this method for practical utility and mass production, it is necessary that the following matters be duly considered.
(1) To keep the difference of temperature between the two substrates constant, the method requires the distance between the substrates to be exactly adjusted. Particularly after forming a film on a given wafer is completed and before forming a film on a subsequent wafer is started, the distance between the substrates must be readjusted. Thus, the process of production is complicated.
(2) The method requires as a source material therefor at least a flat and smooth substrate having the same surface area as the substrate for growth. This requirement poses a serious obstacle to the use of a wafer of a large diameter as the substrate for growth.
(3) The work of simultaneously performing a film-forming treatment on a plurality of substrates for growth entails great difficulty.
(4) The method does not permit use of a blocklike substance as the source material.